If you will recall, the pressure of the atmosphere may
be measured in pounds per square inch (psi) or in atmospheres (atm). There
are 14.7 pounds of air on every square inch at sea level, or 1 atmosphere.
Again, the pressure is caused by billions of moving molecules bombarding each square inch. Most of those molecules are nitrogen (78%), and the rest
are oxygen (21%).

A diver is ready to enter the water and is standing at
the beach with the ambient (surrounding) pressure at 1 atm due to the 15 miles
of air overhead. He or she enters the water and descends to 33' (34' in fresh
water) where the pressure is 2 atmospheres (29.4 psi). One of those atmospheres
is caused by the air and the other from the water. Divers refer to the total
pressure (2 atm.) as, "absolute pressure." When one refers to the pressure
caused only by the water it is called, "gauge pressure." Therefore, the absolute
pressure at 33' in seawater is 2 atm, and the gauge pressure is 1 atm. because
the air is ignored. Likewise, the gauge pressure at 66' would be 2 atmospheres
and the absolute pressure would be 3.

The average male human being has about 2800
square inches on the outside of their body. If one considers there are 14.7
pounds of molecules pounding each square inch, the total pressure works
out to be 21 tons. We should be crushed to death. Fortunately there are molecules
inside our body that push out with an almost equal force. We are stabilized
and don't even know it. In reality, the internal molecules found in the blood
and tissues push with a greater force outward because of the power of the pump
(heart). That is why the flow is outward when a knife wound occurs. (Things
flow from high pressure to low.) Blood pressure, like 120/80, is measured in
millimeters of mercury. Sea level pressure is 760 mm, which equals 14.7 psi.
So, 120 mm is about 2.4 psi and 80 mm is 1.6 psi. When your heart pumps it creates
a pressure of 2.4 psi, and when it is "resting" the pressure drops
to 1.6 psi.

While we are at it, let's see how depth under
the sea is related to millimeters of mercury (Hg). At sea level the average
atmospheric pressure is 29.91" Hg. That is equivalent to 760mm or 76.0cm.
(Check a ruler that has inches next to mm or cm just to see the 760mm is next
to 29.91".) If 33' of seawater is 1 atmosphere then 33' of seawater equals
760mm Hg. It follows that 1' of seawater would equal 23mm Hg (760/33=23),
and the heart pressure at its maximum equals about 5' of seawater.

Now, if you filled a plastic bag with water and took
it down in the sea there would be little change to the shape of the bag.
As you found out with the hypodermic syringe, fluids are barely compressible.
If you filled that same bag with air it would get smaller as you descended
into greater pressure. In fact, the bag would be 1/2 the size at 33' because
the pressure there is double (2 atm absolute). If you went further down to
66', where the pressure is 3 atm., the bag would be 1/3 of the size, and
so on.

Robert Boyle stated the above in a mathematical
way: P1V1 = P2V2. That's Boyle's Law and is very
important for divers. The affect of Boyle's Law can kill a diver. How does the
law work? The P1 is the pressure at the first location such as sea level (1
atm.). The P2 is the pressure at the 2nd location such as 33'. The V1 is the
volume of the gas space, such as our plastic bag, at the first location, and
the P2 is the volume of the gas space at the 2nd location. To make it clearer,
let's say the plastic bag at the beach is 1 liter in size. When you take it
to 33' it should be 1/2 liter. Check it out doing Boyle's math:

1 Atm (at the surface) x 1 liter
should = 2 Atm (at 33') x 1/2 liter

1
x 1 = 2 x 1/2

If you didn't know one of the numbers in the above equation
you should be able to figure it out. If you didn't know how big the bag would
be at 33' you would have math that looked like this:

1 Atm (beach) x 1 liter = 2 Atm (33')
x WHAT ANSWER WOULD GO HERE?

1 x 1=2 x ?
The question mark is a number you should now be able to figure out.

Your body is composed mainly of fluids and solids. If
we were made totally of fluids and solids scuba diving would present far
fewer problems. We have gas spaces in our bodies and they act like a plastic
bag filled with air when we go down to greater pressures. That is why your ears begin to hurt when you go to the bottom of a swimming
pool. There are gas spaces in the ears that are being squeezed smaller as
one goes deeper. This crushing effect causes discomfort and pain. There are
other gas spaces that also shrink and expand as the diver goes down and up.
The list includes the middle ears, sinuses, stomach, intestines, and the
lungs. The mouth, nose, and throat are open to the outside and are not usually
affected by Boyle's law.

It is important in skin and scuba
diving to keep the pressure inside body air spaces the same as the pressure
on the outside!

Pressure underwater increases
and decreases most rapidly when you are near the surface. Going down 10 feet
in a swimming pool results in a much greater pressure change then going from
30 to 40 feet underwater. Descending from the surface to 33' changes the pressure
from 1 atm. to 2 atm. and that would double it. Descending another 33' to 66',
the pressure would increase from 2 to 3. That is not another doubling. You would
have to go from 33' (2 atm) to 99' (4 atm), or 66', to equal what happens from
the surface (1 atm) to 33' (2 atm). So, the upper 33' doubles the pressure
but it takes 66' to do the same thing when deeper.

As a diver descends the
pressure of the breathing air gets greater. As stated before, the pressure of
the air entering the diver's mouth at 33' is double that of the air breathed
at the surface. There are twice as many molecules of oxygen and nitrogen going
in and out of the diver's lungs. If the diver descends further the number of
molecules increase and that makes breathing more difficult. It is similar to
sucking water verses oil through a straw. At 132' the density of the air is
five times the surface density. Five times the number of molecules must move
out from the tank and through the regulator. Five times the number will enter
the diver's lungs with each breath. Five times the number will be exhaled. The
extra effort may be quite noticeable.

You should be able to figure out what the water
pressure is for any depth. From memory, you probably know the answer to the
question, "What is the pressure of the water at 33' (fresh: 34')?" You would
say 2 atmospheres, correct? If the question was for 66" (68'), you would answer
3 atmospheres. Now, what if you were asked for the pressure at 57' would you
be able to figure it out? Try to work out a simply equation for doing this math.
Use 33' and 66' in the search because you already know the answers for those
two. The answer is below (BUT DO NOT LOOK AT IT BEFORE FIGURING THE ANSWER OUT).